1. Field of the Invention
This invention relates to near-equiatomic alloys of Ta--Ru and Nb--Ru which exhibit a shape memory effect.
2. Description of the Related Art
The shape memory effect is observed in alloys which undergo a thermoelastic martensitic transformation. This transformation is characterized by the continuous growth of martensite plates as the temperature is lowered and, comparably, the continuous disappearance of these martensite plates as the temperature is subsequently raised. The reversible nature of this transformation can lead to the many interesting features of the shape memory effect. One effect is superelasticity, which occurs above the transformation temperature and consists of the activation of the martensitic transformation in response to an external stress. Any shape changes produced during the transformation are reversed upon release of the external stress. Below the transformation temperature, the material can exhibit a one-way or two-way shape memory effect The one-way shape memory effect exists when the material is deformed below the martensitic transformation temperature and then reverts to its original shape upon heating to above the transformation temperature. With appropriate mechanical and thermal training of the material, this effect can be modified into a two-way shape memory effect. This two-way effect is a reversible shape change which results during both heating and cooling the material through the transformation temperature range.
The shape memory effect was first discovered in NiTi alloys in the early 1960's by W. J. Buehler, J. V. Gilfrich, and K. C. Weiley, J. Appl. Phys, 34, 1467 (1963) and by W. J. Buhler and W. B. Cross in Wire Journal, 2, 41 (1969). There are now three classes of technically important shape memory alloys: NiTi, Cu--Zn--Al and Cu--Al--Ni. A good review of these materials and their properties is given by J. Van Humbeeck and L. Delaey in their article "A comparative review of the (Potential) Shape Memory Alloys" in The Martensitic Transformation in Science and Technology, ed. E. Hornbogen and N. Jost, DGM Informationsgesellschaft, Germany, p 15 (1989). This article shows that all of these alloys have a typical transition temperature near room temperature (i.e. from -200.degree. C. to 170.degree. C.) and can produce typical strains in the polycrystal of 2-4%.
It is possible to raise the transition temperatures of NiTi alloys by the addition of appropriate alloying elements. Some elements which have demonstrated this effect are Au, Zr, Hf, Pt, and Pd, while additions of Co, Fe, Al, and Mg have been shown to decrease the transition temperature. The elements which display the most pronounced effect in raising the transition temperature are Pd as reported by Khatchin et al, Doklady Akakemii Nauk SSSR, 257, 167 (1981) and Pt as reported by P. G. Lindquist and C. M. Wayman, Engineering Aspects of Shape Memory Alloys, Butterford-Heinemann Ltd, 58 (1990), which can achieve transition temperatures up to 510.degree. C. and 1040.degree. C., respectively, for the binary PdTi and PtTi alloys.
Although there have been a number of earlier publications on near-equiatomic Ta--Ru or Nb--Ru alloys, there has been no mention of the shape-memory behavior in these alloys. Optical microscopy of the phase transformations in these alloys (or at least on the effect of these transformations on previously polished surfaces) was initially performed by Schmerling, Das, and Lieberman in the papers M. A. Schmerling, B. K. Das, and D. S. Lieberman, Met. Trans, 1, 3273 (1970); B. K. Das, M. A. Schmerling, and D. S. Lieberman, Mat. Sci. Eng., 6, 248 (1970); and B. K. Das and D. S. Lieberman, Acta Metall., 23, 579 (1975). Although these articles demonstrate the presence of twinning, they attributed this twinning to both the cubic-to-tetragonal and the tetragonal-to-orthorhombic (the orthorhombic phase is actually monoclinic) transformations. Our more recent study, R. W. Fonda and R. A. Vandermeer, "Crystallography and microstructure of TaRu," Phil. Mag. A, 76 (1) 119 (1997), showed these twins to be due solely to the cubic-to-tetragonal transformation (in the strain-free state).
Examples of these earlier publications on near-equiatomic Ta--Ru or Nb--Ru alloys which are not described above or elsewhere in this patent are R. L. Fleischer, "High-strength, high-temperature intermetallic compounds," J. Material Science, 22, 2281 (1987); P. Greenfield and P. A. Beck, "Intermediate Phases in Binary Systems of Certain Transition Elements," Trans. AIME, 206, 265 (1956); E. Raub, and W. Fritzsche, "Die Niob-Ruthenium-Legierungen," Z Metallk., 54, 317 (1963); E. Raub, H. Beeskow, and W. Fritzsche, "Die Struktur der festen Tantal-Ruthenium-Legierungen," Z Metallk., 54, 451 (1963); D. Bender, E. Bucher and J. Muller, "Structure and Electronic Properties of Niobium-Ruthenium Alloys," Phys. kondens, Materie, 1, 225 (1963); G. F. Hurley and J. H. Brophy, "A Constitution Diagram for the Niobium-Ruthenium System above 1100.degree. C.," J. Less-Common Met., 7, 267 (1964); B. K. Das, E. A. Stern and D. S. Lieberman, "Displacive Transformations in Near-Equiatomic Niobium-Ruthenium Alloys--II. Energenetics and Mechanism," Acta Metall., 24, 37 (1976); T. Tsukamoto, K. Koyama, A. Oota and S. Noguchi, "Superconductivity and transformation of near-equiatomic M--Ru (M=V, Nb, Ta) alloys," Cryogenics, 28, 580 (1988); T. Tsukamoto, K. Koyama, A. Oota and S. Noguchi, "Study of Structural Transformation in Near-Equiatornic M--Ru (M=V, Nb, Ta) Alloys Based on the Electron Theory," J. Japan Inst. Metals, 53, 253 (1989); R. L. Freischer, "Intermetallic Compounds for High-Temperature Structural Use", Platinum Metals Rev., 36, 138 (1992); and K. Otsuka and D. Goldberg, "High Temperature Shape Memory Alloys," in Advances in Science and Technology, 10 Intelligent Materials and Systems, ed P. Vincenzini 55 (1995).
The phase diagrams published by H. Okamoto, "Ru--Ta (Ruthenium-Tantalum)," Binary Alloy Phase Diagram Updating Service, J. Phase Equilib., 12 (3) (1991); B. H. Chen and H. F. Franzen, "Phase Transition and Heterogeneous Equilibrium in the TaRu Homogeneity Range," J. Less-Common Met., 157, 37 (1990); T. B. Massalski, Binary Alloy Phase Diagrams, ed. T. B. Massalski, ASM International, p 2758 (1990); and B. H. Chen and H. F. Franzen, "High temperature X-ray diffraction and Landau theory investigation of phase transitions in NbRu.sub.1+x and RhTi," J. Less-Common Met., 153,L13 (1989), are quite useful in defining the variation in transition temperatures as a function of composition. However, there has been no mention about the possibility of a shape memory behavior in these alloys.
Previous to our examination, there has only been one report on mechanical tests on these alloys (beyond a crude "chisel toughness" test), which was reported by R. L. Fleischer, R. D. Field, and C. L. Briant, Met. Trans. A, 22A, 129 (1991), on compositions of Ta--Ru. In that study, they demonstrated that near-equiatomic Ta--Ru alloys have a room-temperature impact resistance and retain their strengths at elevated temperatures. However, again there has been no mention in this (or other papers) about the possibility of a shape memory behavior in these alloys.
Current commercial alloys such as NiTi, Cu--Zn--Al, and Cu--Al--Ni typically operate near room temperature, and while current research on heavily alloyed NiTi compositions has extended the transition temperatures up to 563.degree. C. and 1040.degree. C. for the (Ni,Pd)Ti and (Ni,Pt)Ti alloys respectively, the Pt alloys are subject to brittleness at higher alloying contents and both alloys are subject to loss of shape memory properties through overheating. NiAl alloys, which have also demonstrated promise for high temperature applications, are currently limited in their application to temperatures below about 300.degree. C. to avoid degradation of properties due to aging. The transition temperatures verified for equiatomic TaRu (1110.degree. C.) according to the present invention are even above these temperatures, confirming TaRu as disclosed herein as the highest transition temperature shape memory alloy yet discovered. There are at present no shape memory effect alloys which operate in the higher temperature regime accessible to the new Ru-based shape memory alloys of this invention.
3. Objects of the Invention
It is an object of this invention to provide a shape memory alloy based on a near-equiatomic composition of ruthenium and either tantalum or niobium.
It is a further object of this invention to provide a shape memory effect alloy that has a higher transition temperature than any other known shape memory alloy.
It is a further object of this invention to provide a shape memory effect alloy based on near-equiatomic composition of ruthenium and either tantalum or niobium in combination with NiTi with significantly increased transition temperature with respect to conventional NiTi alloys.
It is an object of this invention to provide a shape memory effect alloy which has a transition temperature significantly higher than the current commercial alloys such as NiTi, Cu--Zn--Al, and Cu--Al--Ni, which have transition temperatures near room temperature.
It is a further object of this invention to provide a shape memory effect alloy which has a transition temperature higher than 250.degree. C.
It is a further object of this invention to provide a shape memory effect alloy that is not subject to degradation due to overheating.
It is a further object of this invention to provide a shape memory effect alloy that is not subject to degradation due to aging.
These and further objects of the invention will become apparent as the description of the invention proceeds.